1. Field of the Invention
This invention relates to a bump forming method in which conductive balls (represented by solder balls or gold balls) are mounted and formed on the type of semi-conductor package (hereinafter referred to merely as xe2x80x9cpackagexe2x80x9d), such as a BGA (Ball Grid Array) package and a CSP (Chip Size Package or Chip Scale Package), in which solder balls are used as a connecting material for connecting an electronic part to a current board. The invention also relates to an apparatus for performing this bump forming method, and to an electronic part formed by this method.
2. Related Art
As shown in FIGS. 65A and 65B, in a package such as a BGA package, an LSI chip 2005 is mounted on one side or face of a board 2001, and terminals of the LSI chip 2005 are connected respectively to terminals of the board 2001 by lead wires 2006 made of gold or gold-plated aluminum, and the LSI chip 2005 is sealed on the board 2001 by a sealing resin 2002, and then solder bumps 2000 are formed respectively on pads 2003 exposed through a resist film 2004 formed on the other side of the board 2001.
One method of forming these solder bumps 2000 is disclosed in U.S. Pat. No. 5,284,287. This U.S. Patent discloses a technique in which solder balls, vacuum picked up by an arraying jig (suction jig), are immersed in a flux reservoir, thereby applying a flux to the solder balls, and these solder balls are mounted respectively on pads (connection terminals) formed on an electronic circuit board, and the electronic circuit board, holding the solder balls by the adhesive force of the flux, is heated (reflowed), thereby forming the solder bumps.
U.S. Pat. No. 5,279,045 also discloses a method of forming solder bumps. This U.S. Patent discloses a technique in which solder balls are supplied to an arraying jig (suction jig) by a gas flow, and the solder balls are mounted on a semiconductor device, and then are heated, thereby forming bumps.
FIGS. 66 to 68 show one example of a conventional solder ball mounting machine. Blocks 2 are fixedly mounted on a base 1 in parallel relation to each other. Linear guides 3 are fixedly mounted respectively on the blocks 2 in parallel relation to each other. A beam 4 is movably supported on the linear guides 3. A feed screw 5 is rotatably supported on the block 2, and is threadedly engaged with a nut (not shown) fixedly mounted on the beam 4. A motor 6 is supported on the base 1, and is connected to the feed screw 5. Therefore, when the motor 6 is operated, the beam 4 is moved in a Y direction (upward and downward in FIG. 66).
Linear guides 7 are fixedly mounted on the beam 4 in parallel relation to each other. A slider 8 is movably supported on the linear guides 7. A feed screw 9 is rotatably supported on the beam 4, and is threadedly engaged with a nut (not shown) fixedly mounted on the slider 8. A motor 10 is supported on the beam 4, and is connected to the feed screw 9. Therefore, when the motor 10 is operated, the slider 8 is moved in an X direction (right and left in FIG. 66).
Linear guides 11 are fixedly mounted on the slider 8 in parallel relation to each other. A saddle 12 is movably supported on the linear guides 11. A feed screw 13 is rotatably supported on the slider 8, and is threadedly engaged with a nut (not shown) fixedly mounted on the saddle 12. A motor 14 is fixedly mounted on the slider 8, and is connected to the feed screw 13.
Therefore, when the motor 14 is operated, the saddle 12 is moved in a Z direction (upward and downward in FIG. 67).
An arraying jig (suction jig) 15 is fixedly mounted on the saddle 12. This arraying jig 15 is formed into a box-like shape, and a plurality of holes 17 for respectively suction-holding solder balls 16 in the same array as the array of solder balls to be mounted on a package are formed in a lower surface of the arraying jig 15.
A solder ball supply device 19 is fixedly mounted on the base 1, and holds the solder balls 16 therein. This solder ball supply device 19 is formed into a box-shape having an open top, and a plurality of holes smaller in diameter than the solder ball 16 are formed in its bottom surface.
A flux supply device 20 is fixedly mounted on the base 1, and the solder balls 16 are immersed a predetermined depth in a flux 21 in this flux supply device 20, thereby applying the flux 21 to the solder balls 16.
A package 24 on which the solder balls 16 are to be mounted is conveyed by belts 22, and is brought into engagement with stoppers 23 to be positioned.
In this construction, the motor 6 and the motor 10 are operated to position the arraying jig 15 above the ball supply device 19. Then, the motor 14 is operated to move the arraying jig 15 downward until the lower end of the arraying jig 15 covers the open top of the ball supply device 19. Then, compressed air is injected from the bottom surface of the ball supply device 19 to float the solder balls 16 between the arraying jig 15 and the bottom surface of the ball supply device 19. At the same time, vacuum pressure is supplied to the arraying jig 15, thereby drawing the air through the holes (suction holes) 17, so that the solder balls 16 are suction-held in the holes 17 in the arraying jig 15.
Upon lapse of a predetermined time period, the compressed air, injected from the bottom surface of the ball supply device 19, is interrupted, and the motor 14 is operated to move the arraying jig 15, suction-holding the solder balls 16, upward.
Then, the motor 6 and the motor 10 are operated to move the arraying jig 15, suction-holding the solder balls 16, into a position above the flux supply device 20. Then, the motor 14 is operated to move the arraying jig 15 downward until the lower ends (about xc2xc to ⅓ of the diameter of the solder ball) of the solder balls 16, suction-held on the lower surface of the arraying jig 15, are immersed into the flux 21, thereby supplying the flux 21 to the solder balls 16. After the flux 21 is thus supplied to the solder balls 16, the motor 14 is operated to move the arraying jig 15 upward.
Then, the motor 6 and the motor 10 are operated to move the arraying jig 15, suction-holding the solder balls 16 supplied with the flux at their lower ends, into a position above a mounting position where the solder balls are transferred and mounted onto the package 24. At this time, the package 24, positioned in engagement with the stoppers 23, is beforehand located in the mounting position. When the arraying device 15 is located in the predetermined position above the package 24, the motor 14 is operated to move the arraying jig 15 downward, so that the solder balls 16 approach the package 24.
At this time, compressed air is supplied into the arraying jig via a pipe 18. This compressed air is injected from the holes 17, so that the suction-held solder balls 16 are released, and are mounted onto the package 24. The solder balls 16 thus mounted on the package 24 are held on the package 24 by the viscous nature of the flux supplied to the lower ends of the solder balls 16. Then, the motor 14 is operated to move the arraying jig 15 upward, and the arraying jig 15 is further moved so as to suction hold the next hold solder balls 16.
However, the above conventional method has the following problems.
In the above solder ball mounting machine, the operations are effected sequentially, and therefore the time required for one cycle is long (about 15 seconds), and the production ability of the solder ball mounting machine is low.
When the kind of the package is changed, the arraying jig must be changed, and the cost, required for preparing many kinds of expensive arraying jigs, and the time and labor, required for changing the arraying jig, offer a problem. In the steps of the above method, one or more solder balls may be omitted in the arraying jig, and also the package and the arraying jig may be improperly registered with each other, and as a result the required number of solder balls are not all mounted on the package, and the solder balls are improperly registered with the pads when mounting the solder balls on the package, so that the defective products are produced.
In the suction holding of the solder balls, if even only one of many suction holes in the arraying jig fails to suction hold the solder ball, the defective product is produced when the solder balls, suction held on the arraying jig, are mounted onto the package, and therefore it is indispensable for all of the suction holes to suction hold the solder balls. If any of the suction holes fails to suction hold the solder ball, it is necessary to cause the vacant suction hole to suction hold the solder ball by repeating the solder ball suction-holding operation. In order to positively achieve the suction holding of the solder balls so as to minimize this repeated operation, the solder balls need to be supplied uniformly to all of the suction holes by blowing the air, but there is encountered a problem that it is difficult to achieve this with respect to the suction holes arranged in a plane.
Further, if the arraying jig has an increased area, it is difficult to blow the solder balls up over an entire area of the container so as to supply the solder balls to a region near to the suction holes in the arraying jig, and as a result there is encountered a problem that the omission of the solder balls may occur frequently.
As shown in FIG. 69, when the lower ends of the solder balls 16, suction held by the arraying jig 15, are immersed in the flux 21 having good wettability, the flux 21 may wet the suction balls to cover them, and may contaminate the arraying jig 15. Once the flux 21 contaminates the arraying jig 15, the transfer of the solder balls from the arraying jig 15 to the board can not be effected perfectly because of the adhesive force of the flux 21, and therefore there is encountered a problem that each time this transfer is to be effected, the arraying jig 15 must be cleaned.
When the package is warped, those solder balls, brought into contact with the associated pads, can be positively mounted on the board, but those solder balls, held out of contact with the associated pads, drop onto the associated pads, so that misregistration of the solder balls may occur. And besides, as shown in FIG. 70, a pressing force 1314, acting on the solder ball 1302 when mounting the solder ball 1302 onto the board 1311, has a component force 1313, acting perpendicular to the board 1311, and a component force 1312 acting parallel to the plane of the board 1311. The parallel component force 1312 tends to displace the solder ball 1302 from the pad 1309, and because of this force, the solder ball 1302 may be out of registry with the pad 1309. In this case, the bump of a good quality can not be formed.
Further, as shown in FIG. 71, when the flux 1319 adheres to the solder ball 1302 in a misregistered manner, even the solder ball 1302 (indicated in a broken line), mounted on a solder ball mounting position, may be displaced to a position, indicated in a solid line, by a fluid force such as a surface tension of the flux 1319. In this condition, even if the solder ball is melted again by the reflow, the solder ball 1309 can easily move to combine with other solder ball or to be flowed away since the solder ball 1309 is not held in contact with the pad 1309. As a result, the bump of a good quality is formed on the pad 1309, and the resultant bump is defective.
Further, as shown in FIG. 72, if a foreign matter 1316 is present at a region where the rupture of an oxide film 1315 on the solder ball 1302 is started, the solder will not be spread over the pad 1309, and therefore the oxide film 1315 can not be sufficiently removed from the solder ball 1302, so that a defective solder bump, in which the solder ball 1302 is incompletely bonded to the pad 1309, is formed.
It is an object of this invention to provide a bump forming method which overcomes the above problems of the prior art, and achieves a high reliability and a high production ability, and also to provide an apparatus therefor and an electronic part produced by this method and this apparatus.
According to one aspect of the present invention, there is provided a bump forming method comprising the steps of:
causing an arraying jig to vacuum pick up conductive balls;
applying an adhesive liquid, comprising one of a flux, a solder paste and a conductive particle-filled adhesive, to the conductive balls;
registering the conductive balls respectively with pads on an electronic part;
pressing the conductive balls respectively against the pads on the electronic part to mount the conductive balls thereon; and
heat treating the electronic part having the conductive balls mounted thereon.
According to another aspect of the invention, there is provided a bump forming apparatus comprising:
means for causing an arraying jig to vacuum pick up conductive balls;
means for applying an adhesive liquid, comprising one of a flux, a paste and a conductive particle-filled adhesive, to the conductive balls;
means for registering the conductive balls respectively with pads on an electronic part;
means for pressing the conductive balls respectively against the pads on the electronic part to mount the conductive balls thereon; and
means for heat treating the electronic part having the conductive balls mounted thereon.